1. Field of the Invention
The invention relates to a force measuring device having a ring which is coaxial with an axis and is connected to one force input member each on its outer side and on its inner side, can be deformed by forces input into it by the force input members and is connected to a transducer.
Such a force measuring device can serve as a weighing cell to the weigher of a metering means or of another weigher.
2. Description of the prior art
A force measuring device disclosed in German Patent number 3,924,629 has a cell with an axis. The one-piece cell has a ring which is associated with one force input member each on its inner surface and on its outer surface. The ring can be deformed by forces parallel to the axis and transmitted to it by the force input members and is connected to a transducer for converting a force into an electrical signal. The transducer has wire strain gages which are fastened on the two opposite end faces of the deformable ring.
If, when such a force measuring device is used, for example, the annular or jack-like, outer force input member connected to the outer surface of the deformable ring rests on a fixed support and a compressive force directed vertically downward acts on the inner force input member connected to the inner surface of the deformable ring, the ring is deformed. In the ideal case, this deformation is identical along the entire ring circumference. If, however, the compressive force acts slightly eccentrically relative to the axis, the force or force density exerted on the ring also has different magnitudes along its circumference. Accordingly, the ring is nonuniformly greatly deformed along its circumference, with the result that measuring errors may occur. As disclosed in the cited German Patent No. 3,924,629, wire strain gages should furthermore be tightly sealed off from the environment. The force measurements carried out using wire strain gages are moreover very sensitive to changes in the electrical supply or reference voltage and to temperature changes. Moreover, such measurements may be influenced by the contact resistances of electrical connections and by any contact voltages generated at such connections. When wire strain gages are used, expensive measures are required in order to permit exact measurements, so that the production of force measuring devices permitting exact measurements is relatively expensive particularly in small numbers.
Force measuring devices having a transducer which possesses at least one string which vibrates during the measurement have also been disclosed. For example, U.S. Pat. No. 5,095,764 discloses transducers having a force transmission device which possesses two laminae arranged a distance from one another. Together with pins connecting them, they form holders which hold the string. If a force to be measured is input into the force transmission device during a measurement, this results in a change in the distance between the two holders.
The transducers of these known force measuring devices having a string are suitable for the measurement of forces which act on the force transmission device and which are relatively small and are, for example, not more than 50 to 100N. However, there is a need for measuring also substantially larger forces. Such a need exists, for example, in force measuring devices which are used in gravimetric metering devices for weighing a storage container.
Various force measuring devices which have a transducer possessing at least one string and one force transmission device which permits a rather large force reduction is also known. U.S. Pat. No. 4,240,289 discloses, for example, force meters comprising a frame and a load support which is guided by means of a parallel guide thereof and is connected by a first string to the frame via a second, weaker spring to the string of the transducer. However, these and other, more or less similar force measuring devices occupy rather a large amount of space. In the case of a force measuring device intended for gravimetric metering means and in the case of many other possible uses of such a device, it is however desirable if the force transmission device used for force reduction and the transducer are as small and compact as possible.